The Great Heathen Army was a coalition of Norse warriors, originating from Denmark, Norway, Sweden who came together under a unified command to invade the Anglo-Saxon kingdoms that constituted England and avenge the death of Ragnar Lothbrok in AD 816.
The Great Army, having set sail from Kattegat, arrives on Northumbrian shores. Bishop Unwan and King Aelle set off to meet them immediately, encountering them close to York. King Aelle is convinced his army will rapidly dispose of the Norsemen as he sees what looks like a few dozen warriors. However, thousands of Vikings emerge from behind the hills and regroup, forming an astonishingly massive invading force. Led by Bjorn and Ivar, the Vikings charge and quickly overrun the Northumbrian forces by sheer power.
The Great Heathen Army manages to move swiftly and quickly across England. The Norsemen soon spread chaos and wreak havoc across the English countryside, especially in Mercia, where many civilians are forced to flee from their advancing foe.The Vikings then disembark near Repton, in the Trent Valley located in Derbyshire. Aethelwulf, King Egbert's son, is informed of their arrival on Mercian soil. Their number is estimated as over 4000 men. Aethelwulf is dispatched with the task of assembling a Saxon army whose goal will be to stop the Norsemen's advance. Aethelwulf assembles a considerable force composed of a few thousand Mercian and Wessex soldiers and immediately moves towards the Vikings positions in order to engage them.
Having been informed by a scout of the progression of the English army, Ivar and Bjorn had already inspected the battlefield and elaborated their strategy. As soon as the English army arrives on the battlefield, the Viking force split in two groups, and Aethelwulf orders his soldiers to follow them. A few hundred Viking warriors then appear behind the pursuing English army, while Bjorn and his force run through the woods in order to trap their foe. The Saxons turn around in order to pursue the fleeing Norsemen but they are met with a rain of arrows coming from the woods. The Saxon suffer important casualties but manage to form a shield wall. The force led by Ivar and Floki suddenly emerge at the top of a hill, and the weakened Saxon pursue them. Ivar then orders Floki to simulate their retreat, which allows the force to run through the woods and surround the English army. Annoyed of being made a fool, Aethelwulf orders his army to go straight for Repton, where the Viking ships are anchored.
However, the Saxons are yet again met with hundreds of arrows pouring down from the top of a hill on which hundreds of Viking archers are inflicting heavy casualties on the English soldiers. The forces led by Bjorn and King Harald then emerge from the top of the hills and form a massive shield wall in order to block the way to Repton. The battle then engages, the forces colliding into each other with great strength, and the hand to hand combat begins, which turns into the Viking's favor. Bjoin gains of King Ecbert, before his suicide, legitimize their claim over East Anglia, honoring the promises done to Ragnar, many years ago before his death.
Following their victory at Repton, Ivar and his brothers quarrel about the next step to follow. While Ubbe wants to settle to farm the land, Bjorn shares his wish to raid the Mediterranean. King Harald wishes to return back to Norway Sea but Ivar wishes to continue the fight, raid and pillage now that the Saxons are in disarray. An argument between the latter and his brother Sigurd end in the most tragic way when Ivar kills his brother in sudden rage. A few months afterwards, Sigurd's funeral is held Bjorn and Halfdan leave the great heathen army to sail back to the Mediterranean Sea and king Harald leaves the army as well to sail to Kattegat. Floki decides to leave the great heathen army to sail to the unknown, all the vikings say goodbye to Floki by chanting his name.
Ivar, who feels himself to be the sole and rightful leader of the army, convinces his brothers to attack the fortified town of York in Northumbria to make it a Danish stronghold next to the coast thanks to which they could raid deeper in England at any time and attract even more warriors. The Great Heathen Amy attacks the town by surprise on Ascension Day, quickly overrunning the meager garrison before butchering the inhabitants, most notably in the cathedral, where Ivar shows his gruesome dark side by torturing and killing the bishop.The Danes then start reinforcing the defenses of the town in order to make it impregnable, but they leave some sections of the old Roman walls fall into disrepair, which Bishop Heahmund and King Aethelwulf come to know of when they plan to retake the town, following a vision from Alfred.
Heahmund and Aethelwulf start moving north with a powerful force of some 1 500 men, plus some levies. The Saxons attack the town through the undefended Roman walls, which were apparently purposefully neglected by Ivar to trap the Saxon forces in the town. The Saxon forces are caught by surprise when they are met by volleys of Danish arrows and trample to their death over barbed metallic points that were prepared by the Danes in dead end or narrow alleyways. A vicious street fighting, close-quarter combat begins, while Heahmund shows formidable skills. However, the Saxon's loss of morale and their inability to get through a ferocious Danish line proved enough for them to retreat, losing in the process up to a quarter of their troops, while the Danes still lost a few hundred of their own.
Ivar secures his position as the leader of the Great Army. Ubbe leaves for Kattegat with his few followers however, Hvitserk defects to join Ivar. The Saxons lay siege to York, they massacred the hunting parties and blocked the city. the Vikings pretend to burn bodies. Led by Ivar the army go to the sewers of the city, The Saxons find the city deserted but when they are celebrating they are cut short when the army appears from the sewers and engage in battle. The the great heathen army wins the Saxons retreat and they capture bishop Heahmund. Ivar has a conversation with Heahmund about religion and Ivar decides to take the Bishop with him to Kattegat.
Following the succesful conquest and establishment of a stronghold for viking raids in England, the majority of Ivar's followers goes to Norway to expel Lagertha from the throne of Kattegat. The remainder are the warriors of jarl Olavsonn who holds his alligence to Ivar as his overlord. Following the Kattegat Civil War, Ivar's warriors return to their homes while king Harald returns to York to use the city as a base for a raid in Wessex. Jarl Olavsonn is swayed to join Harald effectivly taking control of the active remnants of the great army. After the forces of king Harald are crushed at the Battle of Marton, Harald remains at York to prepare his new move. Still in control of a significant military force, Harald is sought out by Bjorn to depose Ivar. Harald agrees and the remainder of the great army (now absorbed into Harald's army) leaves York for good. The english retake the city, while the great army as an organisation has ceased to operate.